The present invention relates to a roof antenna set up on the roof of a car. In particular, the present invention relates to an antenna which can receive electric waves in three kinds of bands, i.e., a portable, wireless telephone band, a FM radio band, and an AM radio band.
There are various kinds of car antennas. Recently, a roof antenna set up on the roof of a car is popular because setting up an antenna at the highest place enhances the sensitivity. Since a FM radio and an AM radio are generally fixed inside a car, a roof antenna capable of receiving radio waves in both a FM radio band and an AM radio band has been spreading.
As shown in FIG. 12, such a conventional roof antenna 100 capable of receiving waves within two kinds of bands is fixed in the rear part of a car roof. FIG. 13 illustrates a circuit of the roof antenna 100. FIG. 14 shows an external appearance of the roof antenna 100 in detail.
As shown in FIG. 13, a signal received by the antenna element 101 fixed on a car roof is induced into the inside of a car through a matching circuit 102 and input to an amplifying unit 103 disposed in the car. Note that the matching circuit 102 works so that the antenna element 101 matches to a FM radio band.
Then, signals input to the amplifying unit 103 are branched into two kinds of signals, i.e. signals for a FM band and signals for an AM band by a branching filter 104, and both kinds of signals are amplified by amplifiers 105 and 106, respectively. The amplified FM signals are output from an output terminal 107 and induced into a FM receiver. The amplified AM signals are output from an output terminal 108 and induced into an AM receiver.
When a receiver can receive both AM and FM signals, signals which are output from an output terminal 107 or 108 are first mixed, and then, induced into an AM/FM receiver through a cable.
As shown in FIG. 14, this type of roof antenna has an antenna top 114 at the top end of an antenna element 113. The base portion of the antenna element 113 is molded to a basal part 112 of the antenna element. The basal part 112 of the antenna element is attached to a cover 111, thereby the antenna element 113 is supported by an antenna case. The cover 111 has the matching circuit 102 therein. The cover 111 and an antenna base 110 engaged with the cover: 111 constitute the antenna case. The antenna case is attached to a car roof.
A feed cable 116 drawn out of the matching circuit 102 in the antenna case is taken in the amplifying unit 103 disposed in a car. In the amplifying unit 103, waves are branched and amplified as shown in FIG. 13. The amplified FM signal is induced into a FM receiver through the first cable 117, and the amplified AM signal is induced into an AM receiver through the second cable 118.
When a receiver can receive both AM and FM signals, both AM and FM signals are first mixed, and then, induced into an AM/FM receiver through a cable.
In the conventional roof antenna, the antenna case has only a matching circuit 102. The reason why the amplifying unit 103 is disposed outside of the antenna case is that the size of a projecting part outside a car is regulated in Europe to be not more than 40 mm in height and not less than 2.5 of R(radius of curvature) of the projecting part. This regulation is applied to the size of an antenna case excluding the antenna element, and the size of the antenna case is required to be controlled so as to clear the aforementioned regulation. Therefore, the antenna case could not increase its size, and the amplifier 103 could not be installed in the antenna case. In addition, a large projecting part generally mars the beauty of the external appearance of a car.
As described above, the cover 111 has the matching circuit 102 therein. In order to connect the matching circuit 102 to the antenna dement 113 electrically, a metal fitting for connecting an antenna element is arranged on top of the cover 111. The basal part of the antenna element is fixed on the cover 111 by means of the metal fitting. The metal fitting is connected with the matching circuit 102 inside the cover 111. Thus, the antenna element 113 is connected to the matching circuit 102 electrically.
FIGS. 15 (a) (b) (c) shows the structure of connecting the aforementioned metal fitting with the matching circuit 102 as conventional examples.
FIG. 15(a) shows that one end of a connecting piece 121 consisting of lead wire is connected, by soldering, to a substrate 119 having the matching circuit 102. The other end of a connecting piece 121 is connected to the bottom surface of the metal fitting 120. A soldering iron is inserted into the space between the cover 111 and the substrate 119 for soldering. The lead wire as a connecting piece 121 is long enough to leave a margin because the soldering is conducted before the cover 111 covers the antenna base 110.
FIG. 15(b) shows that one end of a connecting piece 121 is connected, by soldering, to a substrate 119 having a matching circuit 102. The other end of the connecting piece 121 is subjected to screw cutting. Then, the cover 111 is fixed to the antenna base 110, followed by screwing the connecting piece to the metal fitting 120 from the central hole of the metal fitting 120. Thus the metal fitting 120 for connecting an antenna element is electrically connected with the connecting piece 121.
FIG. 15(c) shows that one end of a connecting piece 121, which is made of elastic metal, is connected, by soldering, to a substrate 119 having a matching circuit 102. When the cover 111 having the metal fitting 120 engages with the antenna base 110, the other end of the connecting piece 121 contacts with the bottom of the metal fitting 120 so as to obtain electrical connection.
However, a conventional roof antenna has some problems, i.e., a conventional roof antenna requires the space for storing the amplifying unit, and the setup of the amplifying unit or the like requires complex handling.
Further, since a conventional roof antenna cannot receive electromagnetic waves in a wireless telephone band, a car loaded with a wireless telephone requires another antenna.
Furthermore, many problems arise in the structures shown in FIG. 15(a)(b)(c). As for the structure in FIG. 15(a), when the connecting piece is soldered to the metal fitting, it is prone to melt the cover made of synthetic resins. In addition, this structure requires that the connecting piece is long enough to leave a margin because the soldering is conducted before the cover is fixed to the antenna base. The structure has a problem that the length of an antenna up to a matching circuit is not regularized.
As for the structure in FIG. 15(b), moisture penetrates into the cover through the hole for a positive screw and corrodes the substrate and the like. As for the structure in FIG. 15(c), the metal fitting and the connecting piece are electrically connected by only contacting with each other. Therefore, the contact is unstable and prone to be disturbed by moisture penetrating into the cover for some reason.